"It is with great sadness that Mark and Carol Thatcher announced that their mother Baroness Thatcher died peacefully following a stroke this morning," Lord Tim Bell said on Monday, referring to Thatcher's children.
Britain's Queen Elizabeth II was saddened to hear of her death, Buckingham Palace said.
"The Queen was sad to hear the news of the death of Baroness Thatcher. Her Majesty will be sending a private message of sympathy to the family," it said.
The former ruler, who led Britain from 1979 to 1990, suffered from dementia and has appeared rarely in public in recent years. She was last in hospital in December for a minor operation to remove a growth from her bladder.
Thatcher retired from public life after a stroke in 2002 and suffered several smaller strokes after that. Her husband died in June 2003.
Though her doctors advised against public speaking, a frail Thatcher attended Reagan's 2004 funeral, saying in a pre-recorded video that Reagan was "a great president, a great American, and a great man. And I have lost a dear friend".
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